1. Field of the Invention
This invention relates to local device for a control information transfer system in which a central device and the local devices are interconnected through a control information bus.
2. Description of the Prior Art
In a control information transfer system in which a central device, (for instance, a channel control unit), and a plurality of local devices, (for example, channels), are respectively interconnected through a control information bus, either the common control information bus in serially-connected from the channel control unit to the respective channels, or the channel control unit is connected in parallel to the channels through information buses equal in number to the channels.
Generally, a small information processing system employs the control information transfer system of the type in which the channels are serially-connected with the common control information bus, and a large information processing system employs a system in which the channel control unit and the channels are individually interconnected with the same number of control information buses as the channels.
Since the control information transfer system of the small information processing system is desired to be provided at a low cost effected by reducing the number of control information buses used, the serially connected bus configuration is provided, said configuration requiring a smaller amount of cable for the information buses, and hence is more economical. On the other hand, in the control information transfer system of the large information processing system, it is desired to increase the throughput in information transfer, so that the parallel connected bus configuration is employed although the number of control information buses used increases. In the case of the serially connected bus, there is the limitation that control information cannot be sent from a channel over the bus until the use of the bus is permitted, because there is the possibility that the bus may be used by another channel which is permitted to use it. However, the parallel connected bus configuration is free from such a restriction and a desired one of the buses can be used immediately.
In the past, local devices of different structures have been produced to comply with the two different bus configurations described above. However, it is disadvantageous in terms of manufacturing cost and mass-productivity to prepare local devices having two different structures.